Early commercial aircraft include two sets of mechanically linked controls for pilots sitting in the left and right (or front and back) seats of the flight deck. These mechanical control systems include cables connected between the pilots' controllers and the aircraft control surfaces, such as the aircraft ailerons and elevators. The controllers for the two pilots are mechanically linked so that each pilot feels the effect of the other pilot's input, and the resultant force applied to the control surfaces corresponds to the sum of the forces applied by each pilot.
More modern aircraft and aircraft flight simulators include fly-by-wire systems in which the controller position or the force each pilot applies to a controller, such as a yoke or control stick, is translated into an electrical signal which directs a servo actuator to move the aircraft control surface. A servo motor coupled to the controller moves the controller to a position that corresponds to the amount of force applied by the pilot. The motor also provides resistance to the pilot's force on the controller. In one conventional arrangement shown in FIG. 1A, a system 140 includes a dynamic response model 141 that receives an input pilot force signal F and outputs a controller position demand signal D. A servo motor system 170 receives the position demand signal D and actuates the controller, moving it to position P. A summing junction 144 provides a feedback loop that accounts for friction (calculated by a friction model 142) and controller feel (calculated by a feel profile module 143) to produce characteristics at the controller that mimic those produced by a mechanical system.
FIG. 1B illustrates a typical feel profile which illustrates feedback force as a function of position for a typical feel profile module 143. The feedback force (i.e., the resistance the pilot feels when exerting a force on the controller to change a position of the controller) increases rapidly for small deflections, until the pilot surpasses a breakout point 123. The profile then has a generally smooth, curved shape until the pilot approaches a stop point 124, beyond which any additional attempts to change the controller position result in a very large feedback force.
FIG. 2A illustrates another existing system 240 that receives the pilot force input signal F, applies an inverse feel profile model 243 to produce a static controller position, then applies a second order dynamic response model 241 to produce the final controller position demand signal D. FIG. 2B illustrates an associated inverse feel profile curve having a breakout point 223 and a stop point 224.
One challenge associated with fly-by-wire systems has been to link the controllers for left and right seat pilots in a manner that simulates a mechanical system. For example, FIG. 3 illustrates an existing arrangement having linked control modules 360 (shown as a left control module 360a and a right control module 360b). Each control module 360 receives an input force signal F, applies a dynamic response model 341, provides feedback generated by a friction model 342 and a feel profile model 343 and implemented by a summing junction 344 to produce a corresponding controller position demand signal D. Each control module 360 further includes a linking function 345 that inputs the output controller position demand signals and provides additional feedback via the summing junction 344.
FIG. 4 schematically illustrates yet another existing arrangement having dual control modules 460 (shown as modules 460a and 460b) that each receive a pilot force input F and, via a lookup table 446, produce a position demand signal 452, which corresponds to a static position for the corresponding controller. The position demand signals 452 are linked via linking functions 445 and the resulting signals are passed to stick systems 441 which actuate the controllers to position P. Systems having an arrangement generally similar to that shown in FIG. 4 are available from Stirling Dynamics, Ltd. of Clifton, Bristol, England.
One drawback with at least some of the foregoing systems is that they may not accurately simulate the feel and behavior of mechanically linked pilot controls. For example, some of the foregoing systems may not accurately simulate the feel that one pilot has upon seizing the controls when the other pilot has already exceeded the breakout point. Furthermore, these foregoing systems may not accurately simulate the result obtained when two pilots provide large, opposite inputs. Yet another drawback with some of the foregoing systems is that they may not be versatile enough to allow the simulated link between pilot input forces to be easily updated to take advantage of simulation improvements.